Semiconductor memory

ABSTRACT

A dynamic RAM integrated circuit of the one-element memory cell type is provided with a plurality of data lines, a sense amplifier, a plurality of word lines disposed in a manner to intersect with the data lines, and memory cells disposed at the points of intersection between the data lines and the word lines. The RAM includes a P-type semiconductor substrate and an N-type well region formed in the substrate. The memory cells are disposed within the well, and the sense amplifier, which is connected to the data lines, is constructed of a pair of N-channel MOSFETs formed in the semiconductor substrate and a pair of P-channel MOSFETs formed in the well region.

This is a division of application Ser. No. 377,958, filed May 13, 1982.

BACKGROUND OF THE INVENTION

This invention relates to a semiconductor memory. More particularly, itrelates to a semiconductor memory which is constructed of MISFETs(Metal-Insulator-Semiconductor Field Effect Transistors) represented byMOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors)(hereinbelow, abbreviated to "MOS").

Hereunder, a P-channel MOSFET and an N-channel MOSFET will berespectively called "P-MOS" and "N-MOS", and a complementary MOSFET withboth the MOSFETs combined will be called "CMOS". A pair of data linesconnected to a sense amplifier to be in parallel with each other will bereferred to as "folded data lines".

The so-called one-element memory cell which is constructed of one MOSFETand one capacitor requires only a small number of elements and wirings,and therefore has the merit that the area occupied by the memory cell isquite small compared to other memory cell arrangements. For this reason,semiconductor memories employing one-element type memory cells are beingput into extensive use for dynamic RAMs having a large memory capacity.

Since the performance of a sense amplifier in the dynamic RAM is greatlyinfluential on the operation margin of the memory, the design of thesense amplifier is extremely important in the circuit design of aone-element type memory cell system. In addition, the sense amplifiermust be efficiently formed in one semiconductor substrate together withthe memory cells using semiconductor integrated circuit technology insuch a manner that it will not occupy a large area.

On the other hand, in the dynamic RAM, the structure of the memory cellmust be arranged to prevent soft errors which can result fromα-particles (alpha-particles). Further, when integrated to be unitarywith the sense amplifier, the memory cell is required to have high speedinformation read-out capabilities.

SUMMARY OF THE INVENTION

The principal object of this invention is to enhance the performance ofa dynamic RAM which is constructed of one-element type memory cells,owing to a specific combination between the structure of a senseamplifier and the structure of the memory cell.

Another object of this invention is to provide a semiconductor memorywhich can reduce the probability of soft errors ascribable toα-particles (alpha-particles).

Another object of this invention is to provide a sense amplifier inwhich output potentials close to respective predetermined power sourcevoltages are produced for both read-out information of logic "1" andread-out information of logic "0" in stable states during sensingoperations.

Another object of this invention is to provide a semiconductor memory inwhich the aforecited stable operating sense amplifier and memory cellshaving a low susceptibility to soft errors due to α-particles arefabricated by an identical manufacturing process.

Another object of this invention is to provide a semiconductor memorywhich can raise the read-out speed of information from memory cells andwhich can reduce the power dissipation.

Another object of this invention is to provide a novel semiconductormemory capable of reducing noise in which the aforecited stableoperating sense amplifier is connected to folded data lines.

Another object of this invention is to provide a small-sizedsemiconductor memory in which a complementary sense amplifier isconnected to folded data lines so as to make the chip layout efficient.

According to the fundamental construction of this invention, the memoryarray or memory mat of one-element type memory cells is formed in a wellregion disposed in a semiconductor substrate, and a sense amplifier isarranged in a CMOS circuit form, some of the circuit elements of thesense amplifier being also formed in the well region in which the memorymat is formed, or in another well region.

The characterizing features of a dynamic RAM according to this inventionreside in the following:

(1) In accordance with this invention, there is provided a semiconductormemory in which a plurality of N-type well regions formed by anidentical process are disposed in a P-type semiconductor substrate andin which P-channel MISFETs serving as memory cells and a pair ofP-channel FETs constituting a complementary sense amplifier are formedin the surfaces of the respective well regions. Due to such a memoryarrangement, memory cells having a reduced susceptibility to soft errorsdue to α-particles and a high speed, stable sense amplifier aresimultaneously obtained using conventional complementary MOS ICmanufacturing techniques.

As a result of this, by constructing the memory cell of P-MOSFETs andvarying a word voltage within a range between a power source voltageV_(cc) and (V_(cc) -|V_(thp) |), the selection of information "1" or "0"is permitted, and a memory capable of operating at high speed isobtained.

(2) In accordance with this invention, there is provided a semiconductormemory in which a complementary sense amplifier is connected to foldeddata lines. According to such a memory arrangement, an areal margin inlayout which is approximately double that of prior art arrangementsappears in the pitch direction of data lines. Thus, a high packagingdensity is possible.

(3) In accordance with this invention, there is provided a semiconductormemory which comprises means for precharging folded data lines to apotential intermediate between the logic levels "1" and "0" of a memorycell. According to such a memory arrangement, the change of thepotential of a data line equal to half of the potential differencebetween the logic levels "1" and "0" determines a read-out time, so thata memory of high speed and low power dissipation is obtained.

In addition, the coupling noise of a word line and the data line arecancelled because plus and minus noises develop in the folded data linesrespectively.

Further, since the data line is precharged to a reference potentiallevel which is intermediate between the logic levels "1" and "0" of thememory cell, a dummy cell can be dispensed. This permits significantreductions in chip size.

(4) In accordance with this invention, the starting times of thepositive feedback operation of the P-channel FET pair of a senseamplifier and the positive feedback operation of the N-channel FET pairthereof are made different, so that through-current does not flow. As aresult, a memory of low power dissipation is obtained.

(5) In accordance with this invention, there is provided a semiconductormemory in which the P-channel FET pair and N-channel FET pair of acomplementary sense amplifier are arranged at both the ends of a memoryarray. According to this arrangement, the layout within a chip can beseparated into the group of P-channel FETs and the group of N-channelFETs. This permits efficient packaging.

(6) In accordance with this invention, the folded data lines are formedof aluminum (Al), so that a very low wiring resistance is exhibited.This enhances the operational reliability.

(7) In accordance with this invention, there is provided a semiconductormemory in which an N-type well region for forming memory cells thereinis made as an epitaxial structure. According to such an arrangement, auniform well having a desired impurity concentration can be obtained. Asa result, the threshold voltage can be better controlled and thejunction capacitance can be made lower than in a case where diffusion isused. This improves the high speed capabilities of the memory. Inaddition, the surface impurity concentration of the well can be madelower than in the case of diffusion, so that a memory having a highbreakdown voltage is obtained.

(8) In accordance with this invention, there is provided a semiconductormemory in which a plurality of N-type well regions are formed with wellbiasing wirings in parallel with data lines. According to such a memoryarrangement, well voltages become substantially uniform and wellresistances can be made low. This improves the immunity of the memory tonoise.

(9) Finally, in accordance with this invention, there is provided asemiconductor memory in which the well region forming the memory cellsand the well region forming the sense amplifier are isolated. Accordingto such memory, noise which is developed in the sense amplifier has noinfluence on the memory cells. Thus, a highly reliable operation ispermitted.

Hereunder, the embodiments of a dynamic RAM according to this inventionwill be described with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a memory system utilizing a dynamic RAM (D-RAM)array according to this invention; FIG. 2 is a block diagram of a D-RAMfrom the array shown in FIG. 1;

FIGS. 3A and 3B are timing diagrams of the D-RAM of FIG. 2;

FIG. 4A is a block and schematic diagram of a D-RAM embodying thisinvention;

FIG. 4B is a timing diagram of the D-RAM of the embodiment of thisinvention shown in FIG. 4A;

FIG. 4C is a block and schematic diagram of a D-RAM according to anotherembodiment of this invention;

FIG. 4D is a timing diagram of the D-RAM of the embodiment of thisinvention shown in FIG. 4C;

FIG. 4E is a block and schematic diagram of a D-RAM according to anotherembodiment of this invention;

FIG. 5A is a circuit arrangement diagram of a 2-mat type 64 Kbit D-RAMaccording to this invention;

FIG. 5B is a timing diagram of the 2-mat type 64 Kbit D-RAM shown inFIG. 5A;

FIG. 6 is a layout pattern diagram of a 2-mat type D-RAM IC according tothis invention;

FIGS. 7A and 7B are partial layout pattern diagrams of 2-mat type D-RAMICs according to this invention;

FIG. 8A is an element structure view of a memory cell according to thisinvention;

FIG. 8B is an element structure view of a dummy cell according to thisinvention;

FIG. 9A is a layout pattern diagram of a memory array according to thisinvention;

FIG. 9B is a pattern diagram of a field insulating film in a memoryaccording to this invention;

FIG. 9C is an electrode pattern diagram of a storing capacitor in amemory cell according to this invention;

FIG. 9D is a layout pattern diagram of a memory array and a dummy arrayaccording to this invention; and

FIGS. 10 to 35 are sectional views of a C-MOS dynamic RAM showing therespective steps of a manufacturing process thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments in which this invention is applied to a dynamic RAMemploying one-element type memory cells will now be described withreference to the drawings.

Construction and Operation of the Dynamic Memory System of the Invention

The construction of a dynamic memory system will be explained withreference to FIG. 1. First, a block diagram MS enclosed with a dottedline indicates a dynamic memory system. This system is constructed of aD-RAM IC ARRAY portion in which a plurality of IC chips IC₁₁ -IC_(mB) ofa D-RAM set up in accordance with the present invention are arrayed(hereinbelow, termed "D-RAM"), a central processing unit portion of acomputer (not shown, and hereinbelow, termed "CPU"), and an interfacecircuit IF between the CPU and the D-RAM.

Next, input and output signals between the dynamic memory system and theCPU will be explained. Address signals A₀ -A_(k) are signals forselecting the address of the D-RAM. REFGRNT denotes a refresh grantsignal for refreshing the memory information of the D-RAM. WE denotes awrite enable signal, which is a read-out and write-down instructionsignal for data in the D-RAM. MS denotes a memory start signal forstarting the memory operation of the D-RAM. D₁ -D₈ designateinput/output data on a data bus which couples the CPU and the D-RAM.REFREQ designates a refresh request signal for the memory information ofthe D-RAM.

The dynamic memory system will now be explained separately for the D-RAMand the interface circuit (IF). The D-RAM is made up of an IC arraywhich is constructed according to the present invention into a matrixarrangement of (n×m) words×B bits in such a way that n.kbit integratedcircuits received in individual IC packages (hereinbelow, termed "nk",noting that 1 kbit indicates 2¹⁰ =1024 bits) are arrayed in a number min each column and in a number B in each row.

The interface circuit IF will first be described. Referring, then, tothe interface IF, RAR designates a row address receiver which receivesthe address signals A₀ -A_(i) among the address signals A₀ -A_(k)transmitted from the CPU and which converts them into address signalswith timings adapted to the operation of the D-RAM. CAR designates acolumn address receiver which receives the address signals A_(i+1)-A_(j) among the address signals A₀ -A_(k) and which converts them intoaddress signals with timings adapted to the operation of the D-RAM.

ADR indicates an address receiver which receives the address signalsA_(j+1) -A_(k) among the address signals A₀ -A_(k) and which convertsthem into address signals with timings adapted to the operation of theD-RAM.

DCR indicates a decoder which transmits a chip selection control signalfor selecting the chip of the D-RAM (hereinbelow, termed "CS₁ -CS_(m) "where m=2^(k-j)).

RAS-CT represents a RAS control circuit which transmits a chip selectionsignal and a row address introducing signal having timings adapted tothe operation of the D-RAM.

ADM represents an address multiplexer which multiplexes the addresssignals A₀ -A_(i) and A_(i+1) -A_(j) in time sequence and then transmitsthem to the D-RAM.

RSG represents a refresh synchronous generator which determines a timingfor refreshing the memory information of the D-RAM.

RAC represents a refresh address counter which transmits refresh addresssignals R₀ -R_(l) in order to refresh the memory information of theD-RAM.

DBD denotes a data bus driver whose data input and output operationsbetween the CPU and the D-RAM are switched by the WE signal.

C-CT denotes a control circuit which transmits signals for controllingthe above-mentioned RAC, ADM, RAS-CT, DBD and D-RAM.

Now, the actions of the address signals within the dynamic memory systemwill be described.

The address signals A₀ -A_(k) which are transmitted from the CPU aredivided into the two functions of the address signals A₀ -A_(j) and theaddress signals A_(j+1) -A_(k) within the dynamic memory system. Morespecifically, the address signals A₀ -A_(j) are used as the addresssignals of memory matrices within the respective chips of the D-RAM. Onthe other hand, the address signals A_(j+1) -A_(k) become the chipselection signal which, when viewed from the IC chip of the D-RAM,determines whether or not the entire chip is selected.

Herein, in conformity with the matrix within the IC chip of the D-RAM,the address signals A₀ -A_(j) are so designed that the address signalsA₀ -A_(i) are allotted to the selection of the row of the IC chip array,while those A_(i+1) -A_(j) are allotted to the selection of the columnof the IC chip array.

Now, circuit operations within the dynamic memory system will bedescribed.

RAS signals, i.e., RAS₁ -RAS_(m) signals, a RAS_(a) signal and a RAS_(b)signal, are row address strobe signals, while a CAS signal is a columnaddress strobe signal. The address signals A₀ -A_(i) and those A_(i+1)-A_(j) are respectively applied to the ADM through the RAR and CAR. Whenthe RAS_(b) signal has become a certain level, the row address signalsA₀ -A_(i) are transmitted from the ADM and applied to the addressterminals of the D-RAM. At this time, the column address signals A_(i+1)-A_(j) are prevented from being transmitted.

Next, when the RAS_(b) signal has become the opposite level to thatmentioned above, the column address signals A_(i+1) -A_(j) aretransmitted from the ADM and applied to the address terminals. At thistime, the row address signals A₀ -A_(i) are prevented from beingtransmitted from the ADM.

In this way, the address signals A₀ -A_(i) and A_(i+1) -A_(j) areapplied to the address terminals of the D-RAM in time sequence inaccordance with the levels of the RAS_(b) signal.

Since a refresh control signal R_(cs) is applied to neither the ADM northe RAC, the refresh address signals R₀ -R_(l) are not transmitted fromthe ADM.

The chip selection signals A_(j+1) -A_(k) are converted through the DCRinto the chip selection control signals CS₁ -CS_(m) (m=2^(k-j)) whichprincipally select the chips within the D-RAM. The converted signals arefurther converted into the RAS₁ -RAS_(m) signals whose timings have beencontrolled by the RAS_(a) signal and which are used as the chipselecting signals and the row address introducing signals.

There will now be explained the operation of setting addresses withinthe chips in each column of the D-RAM.

First, the row address signals A₀ -A_(i) are applied to the addressterminals of all the IC chips of the D-RAM. It is assumed that when oneof the RAS₁ -RAS_(m) signals, for example, the RAS₁ signal hasthereafter become a certain level, the ICs numbering B which are locatedin the uppermost stage are selected. At this time, the row addresssignals A₀ -A_(i) are introduced into the row addresses of the memorymatrix arrays within the IC (IC₁₁, IC₁₂, . . . IC_(1B)) chips. Here, thereason why the row address signals A₀ -A_(i) are applied to the ICs inadvance of the RAS₁ signal is that, when the RAS₁ signal is appliedbefore the row address signals A₀ -A_(i), signals other than the rowaddress signals might be introduced.

Subsequently, the column address signals A_(i+1) -A_(j) are applied tothe address terminals of all the IC chips of the D-RAM. Thereafter, whenthe e,ovs/CAS/ signal delayed with respect to the RAS₁ signal has becomea certain level, the column address signals A_(i+1) -A_(j) areintroduced into the column addresses of the memory matrix arrays withinthe B IC chips of n·k bits located in the uppermost stage. Here, thereason why the column address signals A_(i+1) -A_(j) are applied to theICs in advance of the CAS signal is the same as the aforecited reason.The action of the CAS signal is to distinguish which signals are beingsent between the row address signals A₀ -A_(i) and the column addresssignals A_(i+1) -A_(j).

By the above operation, the addresses within the B nk chips in theuppermost stage of the D-RAM have been set. The ICs other than those inthe uppermost stage of the D-RAM are not selected because the RAS₂-RAS_(m) signals are at the opposite level to the level of RAS₁.

Now, the write-down operation and read-out operation of data in the setaddresses will be explained.

The D-RAM is so designed that the write-down operation and read-outoperation of data are determined by the high level or low level of thewrite enable signal (hereinafter, termed "WE signal"). The write-downoperation is executed in such a way that when the WE signal is at acertain level, data D_(I1) -D_(IB) from the CPU are applied to the setaddresses. The read-out operation is executed in such a way that whenthe WE signal is at the opposite level to the above, the data D₀₁-D_(0B) of the respective addresses having been written down aredelivered at B bits.

Actions of the Control Signals of FIG. 1

In FIG. 1, abbreviations signify the actions of signals. For example, anabbreviation over which an inversion symbol (bar) is placed indicatesthat when the signal is at "0" (Low Level), the action signified by theabbreviation is executed. On the other hand, an abbreviation without thebar symbol indicates that when the signal is at "1" (High Level), theaction is executed.

The C-CT receives instruction signals from the CPU, i.e., the REFGRNTsignal, WE signal and MS signal, and delivers the CAS signal, RAS_(a)signal, RAS_(b) signal, WE signal and R_(cs) signal in response thereto.The actions of these delivered control signals will now be explained.

The CAS signal is the signal for distinguishing which of the row addresssignals A₀ -A_(i) and the column address signals A_(i+1) -A_(j) arebeing transmitted to the respective chips within the D-RAM. It is alsothe signal for introducing the column address signals of the IC chips.

The RAS_(a) signal is the signal for adjusting the timings of the CS₁-CS_(m) signals and supplying the adjusted signals to the IC chip arraywithin the D-RAM.

The WE signal is the signal for determining the read-out of data fromthe memory cells within the IC chips of the D-RAM and the write-down ofdata into the memory cells.

The R_(cs) signal is the signal for starting the refresh operation. Itcan also inhibit the ADM from transmitting the address signals A₀ -A_(i)and A_(i+1) -A_(j) and while simultaneously permitting it to transmitthe refresh address signals R₀ -R_(l) from the RAC.

The RAS_(b) signal is the switching timing signal for converting the rowaddress signals A₀ -A_(i) and column address signals A_(i+1) -A_(j) fromthe ADM into the time-sequential multiplexed signals. In addition, it ismade a signal according to which the time of the switching between therow address signals A₀ -A_(i) and the column address signals A_(i+1)-A_(j) is delayed with respect to the RAS_(a) signal so that the rowaddress signals A₀ -A_(i) can be provided from the ADM when one of theRAS (RAS₁ -RAS_(m)) signals is selected.

Next, the relationship between the WE signal and the data bus driver(DBD) will be described.

The WE signal transmitted from the C-CT is applied to the D-RAM and theDBD. By way of example, when the WE signal is at the high level, theread-out mode is established, and the data of the D-RAM are provided andtransmitted to the CPU through the DBD. At this time, input data arecontrolled by the WE signal so as not to be introduced from the DBD intothe D-RAM. When the WE signal is at the low level, the write-down modeis established, and the input data are applied from the CPU through theDBD to the data input terminals of the D-RAM and are written into theset addresses. At this time, the data outputs of the D-RAM arecontrolled by the WE signal so as not to be provided from the DBD.

Refresh Operation

In the memory cell circuit of the D-RAM, information is retained byaccumulating charges in the MOS capacitor of the one-element type memorycell. However, as is well known, the charges disapear with time due toleakage current. Obviously, this presents the problem that when thecharges of the information "1" (High Level) have reduced to a point ofbeing less than a reference level for discriminating the information "1"and "0", the information "1" is discriminated as "0", resulting in amalfunction. Thus, in order to keep the information "1" stored, thecharges need to be refreshed before they decrease below the referencelevel. This refresh operation needs must be carried out within theinformation storage time of the memory cell. Accordingly, the refreshmode has preference to the read-out mode and the write-down mode.

The refresh operation will now be explained with reference to FIG. 1.

The refresh synchronous generator RSG transmits the refresh requestsignal REFREQ to the CPU every period of (information storagetime)/(number of refresh cycles). The number of refresh cycles isequivalent to the number of word lines connected to a column data line.

Upon receiving the REFREQ, the CPU transmits the refresh grant signalREFGRNT. At this time, neither the write enable signal WE nor the memorystart signal MS is transmitted from the CPU. When the REFGRNT is appliedto the control circuit C-CT, the refresh control signal R_(cs) being theoutput signal thereof is applied to the address multiplexer ADM and therefresh address counter RAC. Then, the ADM responds to the R_(cs) signalto send the address signals R₀ -R_(l), which are exclusively forrefreshing, to the D-RAM in place of the address signals for randomaccess A₀ -A_(j).

There are two broad categories of refreshing methods in the D-RAM. Oneof them is a method in which the respective rows of the IC chip array(IC₁₁, IC₁₂, . . . and IC_(1B) constitute one row) are refreshed byturns. This method has the advantage that the power dissipation requiredfor the refresh operation is generally low. However, it suffers from thedisadvantage that the period of time required or the refresh operationis long.

The other method is a method in which the whole IC chip array of theD-RAM is simultaneously refreshed. This method performs the refresh insuch a manner that, although no illustration is made in FIG. 1, theaddress signals A_(j+1) -A_(k) from the address receiver ADR are appliedto the RAS control circuit RAS-CT without passing through the decoderDCR, so that all the output signals RAS₁ -RAS_(m) of the RAS-CT become acertain level, and the ICs of all the columns of the D-RAM are selectedat the same time. The advantage of this method is that it only requiresa short period of time for the refresh operation. On the other hand, ithas the disadvantage of high power dissipation.

Now, the refresh operation in the matrix array within the IC of theD-RAM will be explained.

When the refresh address signals R₀ -R_(l) are applied from the ADM tothe address terminals of the D-RAM, the RAS signal thereafter becomes acertain level, and the 2^(l+1) row addresses of the IC matrix array aresuccessively selected. At this time, the CAS signal is held at theopposite level to the above. The refresh is accordingly performed insuch a manner that the information of the memory cells connected to theselected row addresses are amplified by a sense amplifier (not shown) soas to expand the level difference between "1" and "0". Since the WEsignal is transmitted to neither the D-RAM nor the DBD during therefresh operation, the input or output operation of data from the DBD isnot executed.

Actions of RAS-Group Signals and CAS-Group Signals

The actions of RAS-group signals (hereinafter, termed "RAS-φ") andCAS-group signals (hereinafter, termed "CAS-φ") will now be explainedwith reference to FIG. 2 which shows a more detailed block diagram ofone of the IC chips from FIG. 1.

(1) RAS-φ Signals:

φ_(AR) is an address buffer control signal, which is a signal that isapplied to an address buffer ADB to determine whether or not levels a₀,a₀, . . . and a_(i), a_(i) corresponding to the row address signals A₀-A_(i) which are latched in the ADB are to be transmitted to a row andcolumn decoder R&C-DCR.

φ_(X) is a word line control signal, which is a signal that is appliedto the R&C-DCR to determine whether or not one selected signal is to betransmitted to a memory array M-ARY in order to select the row addressof the M-ARY.

φ_(PA) is a sense amplifier control signal, which is a signal that isapplied to a sense amplifier SA to drive the SA.

(2) CAS-φ Signals:

φ_(AC) is an address buffer control signal, which is a signal that isapplied to the ADB to determine whether or not levels a_(i+1), a_(i+1),. . . and a_(j), a_(j) corresponding to the column address signalsA_(i+1) -A_(j) which are latached in the ADB are to be transmitted tothe R&C-DCR.

φ_(Y) is a column switch control signal, which is a signal that isapplied to the R&C-DCR to select a column switch C-SW connected to thecolumn data line of the M-ARY, by means of one selected signal.

φ_(OP) is a data output buffer and output amplifier control signal,which is a signal that is applied to a data output buffer DOB and anoutput amplifier OA to transmit read-out data from the M-ARY to anoutput data terminal D_(out).

φ_(RW) is a data input buffer control signal, which is a signal that isapplied to a data input buffer DIB to transmit write-down data from aninput data terminal D_(in) to the M-ARY.

φ_(RW) is a data output buffer control signal, which is a signal that isapplied to the DOB to prevent any data from being delivered to the dataoutput terminal D_(out) during the write-down operation.

Construction and Operation of a D-RAM

The construction of the D-RAM will now be described with reference toFIG. 2. A block enclosed with a dotted line indicates the integratedcircuit IC of the D-RAM constructed in accordance with this invention.In the IC, a block enclosed with a two-dot chain line is a timing pulsegenerating block TGB, which is constructed of circuits for generatingthe signals to control the operations of the various circuits of theD-RAM.

Now, the operations of the various circuits of the D-RAM will bedescribed with reference to FIGS. 3A and 3B.

When the row address signals A₀ -A_(i) have been introduced into theaddress buffer ADB and latched therein, the RAS signal becomes the lowlevel later than the row address signals A₀ -A_(i). This measure ofmaking the RAS signal later than the row address signals A₀ -A_(i) isintended to reliably introduce the row address signals A₀ -A_(i) as therow address in the memory array.

Subsequently, the signal φ_(AR) delayed from the RAS signal is appliedto the ADB, so that the levels a₀, a₀, . . . and a_(i), a_(i)corresponding to the latched row address signals are transmitted to therow and column decoder R&C-DCR. When the levels a₀, a₀, . . . and a_(i),a_(i) have been applied to the R&C-DCR, this R&C-DCR conducts theoperation of allowing only selected ones to remain at the high levelwhile placing unselected ones at the low level.

The selected signals are transmitted to the M-ARY when the signal φ_(X)delayed from the signal φ_(AR) is applied to the R&C-DCR. This measureof delaying φ_(X) with respect to φ_(AR) is intended to operate theR&C-DCR after completion of the operation of the ADB. The row address inthe M-ARY is thus set in such a manner that, since one of the 2^(i+1)output signals of the R&C-DCR becomes the high level, one row addressline within the M-ARY which corresponds to it is selected.

Next, the information as to the "1" and "0" states of the memory cellsconnected to the selected row address line in the M-ARY is respectivelyamplified by the sense amplifier SA shown in FIG. 2. This operation ofthe SA starts upon application of the signal φ_(PA).

Thereafter, when the column address signals A_(i+1) -A_(j) have beenintroduced into the ADB and latched therein, the CAS signal becomes thelow level later than the column address signals A_(i+1) -A_(j). Themeasure of making the CAS signal later than the column address signalsA_(i+1) -A_(j) is intended to reliably introduce the column addresssignals as the column address in the memory array.

When the signal φ_(AC) delayed from the CAS signal is subsequentlyapplied to the ADB, the levels a_(i+1), a_(i+1) . . . and a_(j), a_(j)corresponding to the column address signals are transmitted to theR&C-DCR. Then, the R&C-DCR conducts an operation similar to the above.The selected signals are transmitted to the column switch C-SW shown inFIG. 2 when the signal φ_(Y) delayed from the signal φ_(AC) is appliedto the R&C-DCR. The column address in the M-ARY is thus set in such amanner that, since one of the 2^(j-i) output signals of the AD-DCRbecomes the high level, one C-SW is selected, so a column address lineor data line connected to this C-SW is selected. In this way, oneaddress within the M-ARY is set.

The read-out and write-down operations for the address set as describedabove will now be explained.

In the read-out mode, the WE signal becomes the high level. This WEsignal is designed so as to become the high level before the CAS signalbecomes the low level. The reason for this is that, since the low levelof the CAS signal results in setting one address of the M-ARY, the WEsignal is brought into the high level beforehand, thereby to prepare forthe read-out operation and to shorten a read-out starting time.

When the signal φ_(OP) of the CAS-group signal is applied to the outputamplifier OA, this output amplifier becomes active, and the informationof the set address is amplified and then read out at the data outputterminal D_(out) through the data output buffer DOB. The read-out iscarried out in this manner, and the read-out operation is completed whenthe CAS signal becomes the high level.

In the next place, in the write-down mode, the WE signal becomes the lowlevel. When the signal φ_(RW) which is formed by the WE signal of thelow level and the CAS signal of the low level becomes the high level andis applied to the data input buffer DIB shown in FIG. 2, the DIB becomesactive and the write-down data from the input data terminal D_(in) istransmitted to the set address of the M-ARY, whereby the write-downoperation is carried out. In the meantime, the inverted signal of thesignal φ_(RW), that is, the signal φ_(RW) of the low level is applied tothe DOB, to control the M-ARY so as to prevent the read-out of any dataduring the write-down operation.

Arrangement and Operation of a D-RAM Transistor Circuit

A practicable circuit arrangement of the D-RAM according to thisinvention will now be set forth.

FIG. 4A shows a first embodiment of the circuit arrangement of the D-RAMof the present invention. Hereunder, the present invention will bedescribed on the basis of this embodiment.

1. Construction of Memory Cell M-CEL:

The M-CEL of 1 bit consists of a capacitor C_(S) for storing informationand a P-MOS Q_(M) for selecting an address. The information of logic "1"or "0" is stored in the form of the existence or nonexistence of chargesin the capacitor C_(S). The information of the logic "1" is accumulatedby a voltage V_(cc), and that of the logic "0" by the null potential.

The gate of the P-MOS Q_(M) is connected to the word line WL, and one ofthe source and drain thereof is connected to the data line DL and theother to the capacitor C_(S).

2. Switching Operation of Memory Cell M-CEL:

When the gate voltage of the P-MOS Q_(M), i.e., the word voltage, hasbecome lower than the power source voltage +V_(cc) by a thresholdvoltage V_(thp) (the threshold voltage of the P-MOS Q_(M)), the P-MOSQ_(M) turns "on" to permit the selection of the memory cell M-CEL. Onthe other hand, in a case of using an N-MOS for a prior-art memory cell(not shown) rather than a P-MOS, when the word voltage has been changedfrom 0 (zero) V to (V_(cc) -V_(thn)) (V_(thn) ; the threshold voltage ofthe N-MOS Q_(M)), the N-MOS Q_(M) turns "on" to permit the selection ofthe memory cell.

The switching speed of the P-MOS Q_(M) shown in FIG. 4A can be madehigher than that of the N-MOS in such a prior-art memory cell becausethe information of the logics "1" and "0" can be determined with onlythe magnitude between V_(cc) and |V_(thp) |. The detailed description ofthe switching operation of the P-MOS Q_(M) is contained in JapanesePatent Application Laid-open Specification No. 56-44189 (Application No.54-119403).

3. Construction of the Sense Amplifier:

Sense amplifiers SA₁ and SA₂ are sense amplifiers by which thedifference of potential changes developing on folded data lines DL₁₋₁and DL₁₋₁ during addressing is expanded in a sense period determined bythe timing signals φ_(PA) and φ_(PA) (sense amplifier control signals),and the input and output nodes of which are coupled to the pair offolded data lines DL₁₋₁ and DL₁₋₁ arranged in parallel.

The sense amplifiers SA₁ and SA₂ are connected in parallel, and it canbe considered that both effectively constitute a single sense amplifier.They differ, however, in that the SA₁ is constructed of N-MOS Q_(S7) andQ_(S8), whereas the SA₂ is constructed of P-MOS Q_(S5) and Q_(S6) of theopposite conductivity type. Each sense amplifier is composed of a pairof cross-connected FETs (Q_(S5) -Q_(S8)) for bringing about apositive-feedback differential amplification operation, and a FET(Q_(S4) -Q_(S9)) connected on the source side of the cross-connectedFETs for controlling the positive-feedback differential amplificationoperation.

As noted above, sense amplifiers SA₁ and SA₂ can be effectivelyconsidered as a single complementary sense amplifier, and hence, theymay well be arranged in adjacency to each other. However, they can alsobe disposed to be separate from each other as in FIG. 4A (for example,at both the ends of the M-ARY) for the purpose of efficiently packagingthem in consideration of the disposition and configuration of wirings,the transistors, well regions, etc.

More specifically, the sense amplifier SA₂ and the memory array M-ARYwhich are constructed of the P-MOS, and the sense amplifier SA₁ and aprecharging circuit PC which are constructed of the N-MOS can bedisposed in an isolated manner. Therefore, the circuit dispositionwithin the chip can have the P-MOS portion and the N-MOS portionisolated, and the circuits can be efficiently packaged.

The folded data lines DL₁₋₁ and DL₁₋₁ are fomred of a metal such as Al,Au, Mo, Ta and W. Since the metal is very low in resistivity, voltagedrops in the data lines during the operation are small and nomalfunction occurs.

4. Construction of the Precharging Circuit:

The precharging circuit PC is composed of a pair of N-MOS Q_(S2) andQ_(S3) for precharging the data lines to about half (V_(DP)) of thepower source voltage V_(CC), and an N-MOS Q_(S1) for eliminating theunbalance of precharged voltage between both the data lines. Asindicated by marks * in the figure, these N-MOS are designed so as tohave threshold voltages lower than those of the other N-MOS.

The numbers of the memory cells to be coupled to the respective foldeddata lines DL₁₋₁ and DL₁₋₁ arranged geometrically in parallel with eachother are made equal in order to raise the detection accuracy. Eachmemory cell is coupled between one word line WL and one of the foldeddata lines. Each word line WL crosses the adjoining pair of data lines.Therefore, even when a noise component developing on the word line WL isinduced on the data lines by electrostatic coupling, the induced noisecomponents appear equally on both the data lines and are cancelled bythe differential type sense amplifiers SA₁ and SA₂.

5. Circuit Operation:

The operation of the circuit in FIG. 4A will be described with referenceto an operating waveform diagram in FIG. 4B.

Before reading out the stored signal of the memory cell, when theprecharge control signal φ_(PC) having a voltage higher than the V_(CC)voltage is at the high level, the N-MOS Q_(S2) and Q_(S3) becomeconductive, and the stray capacitances C₀ and C₀ of the folded datalines DL₁₋₁ and DL₁₋₁ with respect to the ground are precharged to about1/2 V_(CC). At this time, the N-MOS Q_(S1) becomes conductivesimultaneously. Therefore, even when the voltages precharged by theN-MOS Q_(S2) and Q_(S3) have become unbalanced, the folded data linesDL₁₋₁ and DL₁₋₁ are short-circuited and are set at the same potential.The N-MOS Q_(S1) to Q_(S3) have the threshold voltages V_(th) set to belower than those of the transistors without the marks * to avoid anyvoltage loss between the respective sources and drains.

On the other hand, the capacitor C_(S) in the memory cell holds apotential substantially equal to zero volt when the written informationis of logic "0", and it holds a potential substantially equal to V_(CC)when the information is of logic "1". The precharged voltage V_(DP) ofthe data lines DL and DL is set to be intermediate between both thestored potentials.

Accordingly, when the read line control signal φ_(X) becomes the highlevel to address the desired memory cell, the potential V_(DL) of thedata line coupled to the memory cell becomes higher than V_(DP) when theinformation of "1", or the voltage V_(CC), has been read out, and itbecomes lower than V_(DP) when the information of "0", or the zeropotential, has been read out. By comparing the potential of the abovedata line and the potential of the other data line which maintains thepotential of V_(DP), it can be determined whether the information of theaddressed memory cell is "1" or "0".

The positive-feedback differential amplification operations of the senseamplifiers SA₁ and SA₂ are started when the FETs Q_(S9) and Q_(S4) havebegun to turn "on" owing to the respective timing signals (senseamplifier control signals) φ_(PA) and φ_(PA). On the basis of thepotential difference given at the addressing, the higher data-linepotential (V_(H)) and the lower potential (V_(L)) change toward thevoltage V_(CC) and the null potential V_(GND), respectively, and thedifference becomes wider. The sense amplifier SA₁, which is composed ofthe N-MOS Q_(S7), Q_(S8) and Q_(S9), contributes to lower the potentialof the data line to the null potential V_(GND), while the senseamplifier SA₂ composed of the P-MOS Q_(S4), Q_(S5) and Q_(S6)contributes to raise the potential of the data line to V_(CC). Therespective sense amplifiers SA₁ and SA₂ operate in the grounded-sourcemode.

When, in this way, the potential of (V_(L) -V_(GND)) has become equal tothe threshold voltages V_(thn) of the N-MOS Q_(S7) and Q_(S8) of thesense amplifier SA₁, the positive feedback operation of the senseamplifier SA₁ ends. In addition, when the potential of (V_(CC) -V_(H))has become equal to the threshold voltages V_(thp) of the P-MOS Q_(S5)and Q_(S6) of the sense amplifier SA₂, the positive feedback operationof the sense amplifier SA₂ ends. Finally, V_(L) reaches the nullpotential and V_(H) reaches V_(CC), and they become stable in a state oflow impedance.

If desired, the sense amplifiers SA₁ and SA₂ may begin operation of thesame time. On the other hand, SA₁ may well begin its operation prior toSA₂, or SA₂ may begin its operation prior to SA₁. In terms of theread-out speed, the highest speed is obtained by operating SA₁ and SA₂at the same time. Since, however, a through-current flows, the powerdissipation increases. On the other hand, the expedient of making theoperation starting times of SA₁ and SA₂ different has the advantage thatthe through-current is avoided and that the power dissipation decreases.Of course, in terms of the read-out speed, this expedient is somewhatinferior to the above.

FIG. 4C shows the circuit arrangement of another embodiment of the D-RAMof this invention. Parts corresponding to those in FIG. 4A are assignedthe same symbols. FIG. 4A differs from the embodiment of FIG. 4A in thatthe positive feedback operation-control means of the sense amplifier SA₁is constructed of N-MOS Q_(S9) and Q_(S10) which are connected inparallel.

The operations of the sense amplifiers SA₁ and SA₂ in FIG. 4C will bedescribed with reference to FIG. 4D. It is presumed for this descriptionthat the folded data lines are charged to about 1/2 V_(CC) in advance.

The FET Q_(S10) of the positive feedback operation-control means of thesense amplifier SA₁ is rendered conductive by a sense amplifier controlsignal φ₁, whereby one of the FETs Q_(S7) or Q_(S8) is renderedconductive to reduce the lower data-line potential (V_(L)) by a smallamount in the direction of the null potential V_(GND). At this time, thehigher data-line potential (V_(H)) does not change because either theFET Q_(S7) or the FET Q_(S8) is nonconductive. The conductance of theFET Q_(S10) is designed to be smaller than that of the FET Q_(S9).

Subsequently, when the FET Q_(S9) has begun to turn "on" owing to thesense amplifier control signal φ_(PA), the sense amplifier SA₁ startsthe positive feedback operation and changes the potential V_(L) towardthe null potential V_(GND). That is, after the difference of potentialsof the folded data lines has been somewhat expanded by the sameamplifier control signal φ₁, the sense amplifier control signal φ_(PA)is applied so as to execute the positive feedback operation of the senseamplifier SA₁. Thus, even when the potential difference of the foldeddata lines is small, it is permitted to be amplified by the senseamplifier SA₁. In other words, the sensitivity of the sense amplifierimproves.

Subsequently, when the FET Q_(S4) has begun to be turned "on" by a senseamplifier control signal φ_(PA) or φ₂, the positive-feedbackdifferential amplification operation of the sense amplifier SA₂ isstarted, and the higher data-line potential (V_(H)) rises toward V_(CC).Eventually, the potential V_(L) of the data line reaches the nullpotential and V_(H) reaches V_(CC), and they become stable in a state oflow impedance.

FIG. 4E shows the circuit arrangement of the D-RAM according to anotherembodiment of this invention. Parts corresponding to those in FIG. 4Aare assigned the same symbols. This FIG. 4E differs from the embodimentof FIG. 4A in that dummy cells D-CEL are connected to the folded datalines.

The dummy cell D-CEL is constructed of a series connection circuitconsisting of a P-MOS Q_(D1) and a P-MOS Q_(D2). The gate of the P-MOSQ_(D1) is connected to a dummy word line. Either the source or drain ofthe P-MOS Q_(D1) is connected to the data line while the other of thesource and drain is connected to one of the source and drain of theP-MOS Q_(D2). The other of the source and drain of P-MOS Q_(D2) isgrounded.

The dummy cell D-CEL does not require a capacitance C_(ds) for storingthe reference potential. The reason is that the reference potential isprecharged in the data lines. The dummy cells D-CEL are fabricated underthe same manufacturing conditions and with the same design constants asthose of the memory cells M-CEL.

The dummy cells D-CEL function to cancel various noise which develop onthe folded data lines during, for example, the write-down and read-outoperations of memory information.

Time-Sequential Operations of a D-RAM Transistor Circuit

The time-sequential operations of the D-RAM transistor circuit will bedescribed with reference to FIG. 4A.

1. Read-Out Signal Quantity:

The read-out of information is carried out in such a way that the P-MOSQ_(M) is turned "on" to couple the capacitor C_(S) to the common columndata line DL. After this, the manner in which the potential of the dataline DL changes in accordance with the quantity of charges stored in thecapacitor C_(S) is sensed. It is presumed that the potential charged inthe stray capacitance C₀ of the data line DL beforehand is half of thepower source voltage, i.e., 1/2 V_(CC). Then, when the informationaccumulated in the capacitor C_(S) is "1" (i.e., the potential V_(CC)),the potential (V_(DL))_("1") of the data line DL becomes V_(CC) ·(C₀+2C_(S))/2(C₀ +C_(S)) when the correponding memory cell is address. Onthe other hand, when the information is "0" (zero V), the potential(V_(DL))₃₇ 0" of the data line DL becomes V_(CC) ·C₀ /2(C₀ +C_(S)).Here, the difference between the logic "1" and the logic "0", that is, asignal quantity to be detected, ΔV_(S) becomes: ##EQU1##

Due to the fact that the memory matrix of high integration density hasno capacitance even when the memory cells are made small, and the factthat a large number of such memory cells are connected to the commondata lines, C_(S) <<C₀ holds, that is, (C_(S) /C₀) has a value almostnegligible relative to 1 (one). Accordingly, the above equation isexpressed by ΔV_(S) ≃V_(CC) ·(C_(S) /C₀), and ΔV_(S) is a very smallsignal.

2. Read-Out Operation:

Precharging Period

The precharging during the read-out operation is the same as theprecharging operation discussed previously.

Row Addressing Period

The row address signals A₀ to A_(j) which have been supplied from theaddress buffer ADB under the control of the timing signal (addressbuffer control signal) φ_(AR) (refer to FIGS. 3A and 3B) are decoded bythe row and column decoder R&C-DCR, and the addressing of the memorycell M-CEL is started simultaneously with the rise of the word linecontrol signal φ_(X).

As a result, the voltage difference substantially equal to ΔV_(S) arisesbetween the folded data lines DL₁₋₁ and DL₁₋₁ on the basis of the storedcontent of the memory cell as stated before.

Sensing

At the same time that the N-MOS Q_(S) 9 has begun to be renderedconductive by the timing signal (sense amplifier control signal) φ_(PA),the sense amplifier SA₁ starts the positive feedback operation andamplifies the detection signal of ΔV_(S) which developed during theaddressing. Simultaneously with the amplifying operation, or after thestart of the amplifying operation, the sense amplifier SA₂ starts thepositive feedback operation due to the timing signal φ_(PA) and recoversthe level of logic "1" to V_(CC).

Data Output Operation

The column address signals A_(i+1) to A_(j) which have been sent fromthe address buffer ADB in synchronism with the timing signal (addressbuffer control signal) φ_(AC) are decoded by the row and column decoderR&C-DCR. Subsequently, the stored information of the memory cell M-CELat the column address which has been selected by the timing signal(column switch control signal) φ_(Y) is transmitted to the commoninput/output lines CDL₁ and CDL₁ through the column switch C-SW₁.

Subsequently, the output amplifier/data output buffer OA & DOB isoperated by the timing signal (data output buffer and outputamplifier-control signal) φ_(OP), and the stored information which hasbeen read is transmitted to the output terminal D_(out) of the chip.During write-down, the OA & DOB is held inoperative by the timing signal(data output buffer-control signal) φ_(RW).

3. Write-Down Operation:

Row Addressing Period

The precharging, addressing and sensing operations are quite the same asin the foregoing read-out operation. Accordingly, irrespective of thelogic value of input write-down information D_(in), the storedinformation of the memory cell into which the information ought to bewritten is read out between the folded data lines DL₁₋₁ and DL₁₋₁. Thisinformation read out is ignored by a writing operation to be statedbelow. Therefore, the operations up to this point of time maysubstantially be considered as a row address selection operation.

Writing Period

As in the read-out operation, the folded data lines DL₁₋₁ and DL₁₋₁which lie at the column selected in synchronism with the timing signal(column switch-control signal) φ_(Y) are coupled to the commoninput/output lines CDL₁ and CDL₁ through the column switch C-SW₁.

Subsequently, complementary writing input signals d_(in) and d_(in)supplied from the data input buffer DIB in synchronism with the timingsignal (data input buffer-control signal) φ_(RW) are written into thememory cell M-CEL through the column switch C-SW₁. In the meantime, thesense amplifier SA is operating. Since, however, the output impedance ofthe data input buffer DIB is low, the information which is to appear onthe folded data lines DL₁₋₁ and DL₁₋₁ is determined by the informationat the input D_(in).

4. Refresh Operation:

The refresh is carried out in such a way that the information stored inthe memory cell M-CEL which is in danger of being lost is read out ontothe column common data line DL. This read-out information is brought toa restored level by the sense amplifiers SA₁ and SA₂, and is thenwritten into the memory cell M-CEL again. Accordingly, the refreshoperation is similar to the operation of the row addressing or sensingperiod which has been explained for the read-out operation. In thiscase, however, the column switch C-SW₁ is made inoperative, and all thecolumns are simultaneously refreshed row by row.

Circuit Arrangement of a 2-Mat Type 64K-D-RAM

FIG. 5A shows a diagram of a D-RAM circuit arrangement which isconstructed in accordance with this invention and in which memory cellsof about 64 Kbits are divided and arrayed into two memory cell matrics(memory arrays M-ARY₁ and M-ARY₂) each having a memory capacity of 128rows×256 columns=32,768 bits (32 Kbits). Principal blocks in this figureare depicted in conformity with the actual geometrical disposition. Therespective blocks have the same constructions as described above, andare assigned the same synmbols. With regard to the circuit arrangementof the memory array, any of the systems shown in FIGS. 4A, 4C and 4E canbe adopted.

The row-series address selection lines (word lines WL) of the respectivememory arrays M-ARY₁ and M-ARY₂ are supplied from row decoders (servingalso as word drivers) R-DCR₁ and R-DCR₂ with 2⁷ =128 possible decodeoutput signals which are obtained on the basis of row address signals A₀-A₆.

A column decoder C-DCR provides 128 possible decode output signals onthe basis of column address signals A₉ -A₁₅. The column selecting decodeoutput signals are common to the right and left memory arrays and upperand lower adjoining columns within each memory array, totaling fourcolumns.

In order to select any one of the four columns, address signals A₇ andA₈ are alloted. By way of example, the signal A₇ is allotted to theselection of the right or left array, and the signal A₈ is allotted tothe selection of the upper or lower column.

It is a φ_(yij) signal generator circuit φ_(yij) -SG that encodes theaddress signals A₇ and A₈ into four combinations, and it is columnswitch selectors CSW-S₁ and CSW-S₂ that switch the columns on the basisof the output signals φ_(y00), φ_(y01), φ_(y10) and φ_(y11) of thegenerator circuit.

In this manner, the decoder for selecting the columns of the memoryarrays is divided into the two stages of the column decoder C-DCR andthe column switch selector CSW-S₁ as well as CSW-S₂. The first aim ofthe division of the decoder into these two stages is to prevent anyuseless blank part from occurring within the IC chip. That is, it isintended that the vertical arrayal pitch of NOR gates which take chargeof a pair of right and left output signal lines of the column decoderC-DCR and which have comparatively large areas is made equal to thecolumn arrayal pitch of the memory cells. Owing to the division of thedecoder into the two stages, the NOR gate has the number of itsconstituent transistors reduced and can have its occupying area madesmall.

The second aim of the division of the decoder into the two stages isthat the number of the NOR gates connected to one address signal line isreduced. This lightens the load placed on the address signal line andenhances the switching speed.

The address buffer ADB processes the eight multiplexed external addresssignals A₀ -A₇ ; and A₈ -A₁₅ into eight complementary pairs of addresssignals (a₀, a₀)-(a₇, a₇); and (a₈, a₈)-(a₁₅, a₁₅), respectively. Itthen transmits them to the decoder circuits at the timings φ_(AR) andφ_(AC) adjusted to the operations within the IC chip.

Circuit Operation of a 2-Mat Type 64-K-D-RAM

The circuit operation of an address setting process in the 2-mat type64-K-D-RAM will be described with reference to FIGS. 5A and 5B.

First, the address buffer control signal φ_(AR) of the row series risesto the high level, whereby the seven complementary pairs of row addresssignals (a₀, a₀)-(a₆, a₆) corresponding to the row address signals A₀-A₆ are applied from the address buffer ADB to the row decoders R-DCR₁and R-DCR₂ through a row address line R-ADL. Following this, the wordline control signal φ_(X) rises to the high level to activate the rowdecoders R-DCR₁ and R-DCR₂. These row decoders then select one of theword lines WL of each of the memory arrays M-ARY₁ and M-ARY₂ and set itat the high level.

Subsequently, the address buffer control signal φ_(AC) of the columnseries rises to the high level, whereby the seven complementary pairs ofcolumn address signals (a₉, a₉)-(a₁₅, a₁₅) corresponding to the columnaddress signals A₉ -A₁₅ are applied from the address buffer ADB to thecolumn decoder C-DCR through a column address line C-ADL. As a result,one pair of output signal lines among the 128 pairs of output signallines of the colunn decoder C-DCR becomes the high level, and the highlevel signals are applied to the column switch selectors CSW-S₁ andCSW-S₂.

Subsequently, when the column switch control signal φ_(Y) rises to thehigh level, the φ_(yij) signal generator circuit φ_(yij) -SG becomesoperable. On the other hand, the complementary pair of signals (a₇, a₇)corresponding to the address signal A₇ and the complementary pair ofsignals (a₈, a₈) corresponding to the address signal A₈ have alreadybeen applied to the φ_(yij) signal generator circuit φ_(yij) -SGrespectively when the address buffer control signal φ_(AR) has becomethe high level and when the address buffer control signal φ_(AC) hasbecome the high level. Accordingly, when the column switch controlsignal φ_(Y) has become the high level, the φ_(yij) signal generatorcircuit φ_(yij) -SG transmits the signals to the column switch selectorsCSW-S₁ and CSW-S₂ substantially concurrently therewith.

In this way, one pair among a total of 512 transistor pairs in thecolumn switches C-SW₁ and C-SW₂ is selected, and one pair of data linesDL within the memory array are connected to the common data line CDL.

D-RAM-IC Layout Pattern of the 2-Mat Type

The D-RAM IC layout pattern of the so-called 2-mat type in which asingle IC chip includes two divided memory arrays will be described withreference to FIG. 6.

The two memory arrays M-ARY₁ and M-ARY₂, each of which is constructed ofa plurality of memory cells, are arranged in the IC chip in a manner tobe spaced from each other. In the central part of the IC chip betweenthe M-ARY₁ and the M-ARY₂, the common column decoder C-DCR is arranged.The column switch C-SW₁ for the M-ARY₁ is arranged between the M-ARY₁and the C-DCR. On the other hand, the column switch C-SW₂ for the M-ARY₂is arranged between the M-ARY₂ and the C-DCR.

The sense amplifiers SA₁ and SA₂ are respectively arranged in the leftend part and right end part of the IC chip in order to prevent them fromoperating erroneously due to noise, for example, signals applied to theC-DCR, and also to facilitate the layout of wirings.

On the left side of the upper part of the IC chip, the data input bufferDIB, a read/write signal generator circuit R/W-SG, a RAS signalgenerator circuit RAS-SG, and a RAS-group signal generator circuit SG₁are arranged. In proximity to these circuits, a RAS signal applying padP-RAS, a WE signal applying pad P-WE, and a data signal applying padP-D_(in) are provided.

On the other hand, on the right side of the upper part of the IC chip,the data output buffer DOB, a CAS signal generator circuit CAS-SG, and aCAS-group signal generator circuit SG₂ are disposed. In proximity tothese circuits are disposed a V_(SS) voltage feeding pad P-V_(SS), a CASsignal applying pad P-CAS, a data signal deriving pad P-D_(out), and anaddress signal A₆ feeding pad P-A₆.

A main amplifier MA is arranged between the RAS-group signal generatorcircuit SG₁ and the CAS-group signal generator circuit SG₂.

A V_(BB) generator circuit V_(BB) -G is arranged above one of thecircuits which occupies a relatively large area, such as the RAS-groupsignal generator circuit SG₁, the CAS-group signal generator circuit SG₂or the main amplifier MA. The reason for this is that the V_(BB) -Gproduces minority carries, and there is the danger that the memory cellsconstituting the M-ARY₁ and M-ARY₂ will suffer from undesiredinformation inversion due to the minority carries. Therefore, in orderto prevent this, the V_(BB) generator circuit V_(BB) -G is disposed at aposition which is as far away as possible from the M-ARY₁ and M-ARY₂ asdescribed above.

On the left side of the lower part of the IC chip, the row decoderR-DCR₁ for the M-ARY₁ is arranged. In proximity to the R-DCR₁ aredisposed address signal feeding pads P-A₀, P-A₁ and P-A₂ and a V_(CC)voltage feeding pad P-V_(CC).

On the other hand, on the right side of the lower part of the IC chip,the row decoder R-DCR₂ for the M-ARY₂ is arranged. In proximity to therow decoder R-DCR₂, there are disposed address signal applying padsP-A₃, P-A₄, P-A₅ and P-A₇.

The address buffer ADB is arranged between the R-DCR₁ and R-DCR₂.

Layout Pattern Diagram of the Power Supply Lines

A partial layout pattern diagram centering on the memory arrays M-ARYand the sense amplifiers SA₁ and SA₂ in the 64 Kbit D-RAM will bedescribed with reference to FIG. 7A. The memory arrays M-ARY and thesense amplifiers SA₂ are formed in individual N-channel type wellregions enclosed with dot-and-dash lines. Since the memory arrays M-ARY,the sense amplifiers SA₂, etc. are laid out in line symmetry withrespect to the column decoder C-DCR, the memory arrays M-ARY and thesense amplifiers SA₁ and SA₂ within well regions on the right side areomitted or illustrated only schematically.

Since the N-channel type wells are supplied with the power sourcevoltage V_(CC), power supply lines V_(CC) -L are formed as shown in FIG.7A. In FIG. 7A, assuming that the memory array M-ARY₁₋₁ is in the firstrow, the power supply line is formed every 32nd row of the memory arraysM-ARY. As the intervals of the power supply lines become larger, thewell voltage becomes less uniform. In order to prevent this drawback,the power supply line may be formed every row of the M-ARY.Unfortunately, if this is done, the chip area increases. It is thereforefavorable to form the power supply lines at equal intervals among thememory arrays M-ARY, for example, at intervals of 8 rows, 16 rows, 32rows or 64 rows.

To the end of making the well voltage uniform, the power supply linesare formed of a metal exhibiting almost no voltage loss, such as Al, Au,Mo and Ta. If the power supply lines formed of the metal are disposed inthe wells, they should preferably be arranged in parallel with the datalines so as not to short-circuit to the data lines formed of Al.

The N-channel type well region for the memory arrays M-ARY and that forthe sense amplifiers SA₂ are isolated in the arrangement shown in FIG.7A for the following reason.

A voltage drop develops between the power supply line within the wellregion for the sense amplifier SA₂ and the positive feedbackoperation-control means (FIG. 4A) within the sense amplifier SA₂. Thevoltage drop becomes greater in the sense amplifier SA₂ as a function ofits increased distance from the power supply line, and this producesnoise. If the memory arrays M-ARY and the sense amplifiers SA₂ areformed within a single N-type well region, the aforecited voltage dropwill lower the well potential. This, in turn, results in lowering thethreshold voltage V_(TH) of the P-MOS Q_(M) (FIG. 4A) of the memorycell. Then, the P-MOS Q_(M) will become easier to turn "on", which canbe the cause of a malfunction in the memory operation. Therefore, inFIG. 7A, the N-channel type well regions for forming the memory arraysM-ARY and the sense amplifiers SA₂ are formed independently of eachother so that noise developing in the sense amplifiers SA₂ is preventedfrom influencing memory operations.

FIG. 7B illustrates a modification of the layout shown in FIG. 7A, andis a partial layout pattern diagram centering on the memory arrays M-ARYand the sense amplifiers SA₁ and SA₂ in the 64 Kbit D-RAM. Partscorresponding to those in FIG. 7A are assigned the same symbols. FIG. 7Bdiffers from the layout of FIG. 7A in that the memory arrays M-ARY andthe sense amplifiers SA₂ are formed within an identical well region.This modification has the advantage that the chip area becomes smallerthan in the layout of FIG. 7A. It is disadvantageous, however, in thatthe noise developing in the sense amplifiers SA₂ is prone to affect thememory operations as described above.

Element Structure of a Memory Cell

FIG. 8A is a perspective sectional view showing the element structure ofone memory cell M-CEL described above. Numeral 1 designates a P-typesilicon semiconductor substrate, numeral 2 a comparatively thickinsulating film such as silicon oxide film (hereinbelow, called "fieldinsulating film"), numeral 3 a comparatively thin insulating film suchas silicon oxide film (hereinbelow, called "gate insulating film"),numerals 4 and 5 P⁺ -type semiconductor regions, numeral 6 a firstpolycrystalline silicon layer, numeral 7 a P-type surface inversionlayer, numeral 8 a second polycrystalline silicon layer, numeral 9 a PSG(phosphosilicate glass) layer, numeral 10 an aluminum layer, and numeral100 an N-type well region.

The MOS Q_(M) in one memory cell M-CEL has its substrate, well region,drain region, source region, gate insulating film and gate electrodemade of the above-mentioned P-type semiconductor substrate 1, N-typewell region 100, P⁺ -type semiconductor region 4, P⁺ -type semiconductorregion 5, gate insulating film 3 and second polycrystalline siliconlayer 8, respectively. The second polycrystalline silicon layer 8 isused as, for example, the word line WL₁₋₂ shown in FIG. 4A. The aluminumlayer 10 connected to the P⁺ -type semiconductor region 5 is used as,for example, the data line DL₁₋₁ shown in FIG. 4A.

On the other hand, the storing capacitor C_(S) in the memory cell M-CELhas one electrode, a dielectric layer and the other electrode thereofmade of the first polycrystalline silicon layer 6, the gate insulatingfilm 3 and the P-type surface inversion layer 7, respectively. Theground voltage V_(SS) is applied to the first polycrystalline siliconlayer 6, and it induces the P-type surface inversion layer 7 in thesurface of the N-type well region 100 owing to the field effect causedthrough the gate insulating film 3.

Although the MOS Q_(M) in the memory cell M-CEL has been exemplified asthe P-channel type, an N-channel type MOS Q_(M) can be formed bychanging all the aforecited conductivity types into the oppositeconductivity types.

Element Structure of a Dummy Cell

FIG. 8B is a perspective sectional view showing the element structure ofone dummy cell D-CEL. In FIG. 8B, especially numerals 11, 12 and 14designate P⁺ -type semiconductor regions, numerals 17 and 18 secondpolycrystalline silicon layers, and numeral 19 an aluminum layer.

The MOS Q_(D1) in one dummy cell D-CEL has its substrate, well region,source region, drain region, gate insulating film and gate electrodemade of the P-type semiconductor substrate 1, N-type well region 100, P⁺-type semiconductor region 11, P⁺ -type semiconductor region 12, gateinsulating film 3 and second polycrystalline silicon layer 17,respectively. The second polycrystalline silicon layer 17 extends overthe N-type well region 100 as, for example, the dummy word line DWL₁₋₂shown in FIG. 4E. The aluminum layer 19 connected to the P⁺ -typesemiconductor region 11 extends over the P-type semiconductor substrate1 as, for example, the dummy data line DL₁₋₁ shown in FIG. 4E.

Although the MOS Q_(D1) and Q_(D2) in the dummy cell D-CEL have beenexemplified as the P-channel type, N-channel type MOS Q_(D1) and Q_(D2)can be formed by changing all the aforecited conductivity types into theopposite conductivity types.

Layout Pattern of the Memory Array

The layout pattern of the memory array M-ARY will now be described withreference to FIG. 9A. The memory array M-ARY shown in FIG. 9A is anarrangement in which a plurality of memory cells M-CEL such as shown inFIG. 8 are arrayed in the N-type well region 100. First, theconstruction of the memory array M-ARY will be discussed.

In order to isolate each of the plurality of memory cells M-CEL from oneanother (where each cell is constructed of the MOS Q_(M) and the storingcapacitor C_(S) in the surface of the N-type well region 100), the fieldinsulating film 2 is formed in a basic pattern shown in FIG. 9B. A novelarrangement is also provided in that the field insulating film 2a isdisposed under a contact hole CH₀ which permits the application of theground voltage V_(SS) to the first polycrystalline silicon layer 6.Accordingly, it is possible to prevent an aluminum-silicon alloy formedin the vicinity of the contact hole CH₀ on the basis of the interactionbetween the aluminum layer and the polycrystalline silicon layer frompenetrating through the insulating film located directly below thecontact hole CH₀ and undesirably reaching the surface of the N-type wellregion 100.

On the field insulating film 2 and the gate insulating film 3, the firstpolycrystalline silicon layer 6 which is used as one electrode of thestoring capacitor C_(S) in the memory cell M-CEL is formed in a basicpattern shown in FIG. 9C. Further, on the first polycrystalline siliconlayer 6, the word lines WL₁₋₁ -WL₁₋₆, each of which is formed by thesecond polycrystalline silicon layer 8 in FIG. 8A, extend in thevertical direction in FIG. 9A. Further, on the polycrystalline siliconlayer 6 which serves as one electrode of the storing capacitor C_(S),the power supply line V_(SS) -L for feeding the ground voltage V_(SS)through the contact hole CH₀ extends in the lateral direction in FIG.9A.

On the other hand, the data lines DL₁₋₁ and DL₁₋₁, each of which isformed by the aluminum layer 10 in FIG. 8A, extend substantially inparallel with the power supply line V_(SS) -L as shown in FIG. 9A. Thedata line DL₁₋₁ is connected to the source region of the MOS Q_(M) inone memory cell M-CEL through a contact hole CH₁, while the data lineDL₁₋₁ is connected to the source region of the MOS Q_(M) in anothermemory cell M-CEL through a contact hole CH₂. In addition, the datalines DL₁₋₂ and DL₁₋₂ extend in the lateral direction in FIG. 9A in thesame manner as the data lines DL₁₋₁ and DL₁₋₁, and they are connected tothe source regions of the MOS Q_(M) in memory cells M-CEL throughcontact holes in predetermined locations.

In order to bias the N-type well region 100 to the power source voltgeV_(CC), the power supply line V_(CC) -L extends at an end of the memoryarray M-ARY in the lateral direction in FIG. 9A and substantially inparallel with the data lines.

Layout Pattern of the Memory Array and the Dummy Array

The layout pattern of the memory array shown in FIG. 4E which includesthe memory array M-ARY and the dummy array D-ARY is illustrated in FIG.9D. Parts corresponding to those in FIG. 9A are assigned the samesymbols. The difference between FIG. 9D and the layout pattern of FIG.9A is that the dummy array D-ARY is added. A dummy cell D-CEL shown inFIG. 9D is constructed in the manner set forth below.

A part of the surface of the N-type well region 100 is covered with thefield insulating film 2, while another part of the surface of the N-typewell region 100 is covered with the gate insulating film 3. The P+-typesemiconductor region 14 is used as the common earth line of a pluralityof dummy cells D-CEL.

The dummy word line DWL₁₋₁ is formed on the field insulating film by thesecond polycrystalline silicon layer 17 in FIG. 8B. The dummy word lineDWL₁₋₁ constructs the gate electrode of the MOS Q_(D1) in the dummy cellD-CEL. On the other hand, a control signal line φ_(dc) -L1 which isformed by the second polycrystalline silicon layer 18 in FIG. 8B inorder to apply the discharge control signal φ_(dc) illustrated in FIG.4E is kept apart from the dummy word line DWL₁₋₁ and is extended inparallel therewith. This control signal line φ_(dc) -L1 constructs thegate electrode of the MOS Q_(D2) in the dummy cell D-CEL. Likewise, thedummy word line DWL₁₋₂ and a control signal line φ_(dc) -L2 extend inparallel with the dummy word line DWL₁₋₁ and the control signal lineφ_(dc) -L1.

The data lines DL₁₋₁, DL₁₋₁, DL₁₋₂ and DL₁₋₂ extend from the memoryarray M-ARY to the dummy array D-ARY as shown in FIG. 9D. The DL₁₋₁ isconnected to the source region of the MOS Q_(D1) in one dummy cell D-CELthrough a contact hole CH₃, and the DL₁₋₂ is similarly connected to thesource region of the MOS Q_(D1) in another D-CEL through a contact holeCH₄.

Manufacturing Process of the C-MOS Dynamic RAM

The manufacturing process of the dynamic RAM of the complementary typehaving the N-MOS and the P-MOS (hereinbelow, termed "C-MOS")construction will now be described with reference to FIGS. 10 to 35. Ineach of the figures, X₁ is a sectional process diagram of section X₁--X₁ in the memory array M-ARY shown in FIG. 9A, while X₂ is a sectionalprocess diagram of the C-MOS circuit portion of the sense amplifier SA₂shown in FIG. 4A.

Step of Forming an Oxide Film

As shown in FIG. 10, an oxide film 102 is formed on the surface of asemiconductor substrate 101. The preferred materials for thesemiconductor substrate 101 and the oxide film 102 are a P-typesingle-crystal silicon (Si) substrate having a (100) crystal face and asilicon dioxide (SiO₂) film.

Step of Selective Removal of the Oxide Film

As shown in FIG. 11, in order to form well regions of the conductivitytype opposite to that of the semiconductor substrate, those parts of theSiO₂ film 102 on the semiconductor substrate 101 which correspond to thewell forming regions are removed. To this end, silicon nitride (Si₃ N₄)film 103 is first formed on selected areas of the surface of the SiO₂film 102 as an etching mask. In this state, the SiO₂ film not coveredwith the Si₃ N₄ film 103 is removed with an etchant.

Step of Selective Removal of the Substrate

As shown in FIG. 12, in order to form the well regions of theconductivity type opposite to that of the semiconductor substrate 101within this semiconductor substrate, the semiconductor substrate 101 isetched down to a desired depth by a wet etching process or a dry etchingprocess by employing the Si₃ N₄ film 103 as an etching mask using knowntechniques.

Step of Forming the N-type Well Regions

As shown in FIG. 13 the single crystal of Si is epitaxially grown usingknown techniques in each of the etched regions within the semiconductorsubstate 101. Simultaneously, the Si crystal is doped with arsenic. Inthis way, the N-type well regions 100 having an impurity concentrationof about 10¹⁵ cm⁻³ are formed on the semiconductor substrate 101.Thereafter, the SiO₂ film 102 and the Si₃ N₄ film 103 on thesemiconductor substrate 101 are removed.

By forming the N-type well regions, certain advantages are achieved. Forexample, when the memory cell is constructed within the N-type wellregion, it is possible to prevent stored information from being invertedby the absorption of α-particles into the capacitor C_(S) of the memorycell, since holes generated in and below the N-type well by theα-particles are reflected by the barrier of a P-N junction. Thus, theinfluence of the holes on the capacitor C_(S) is avoided.

By epitaxially forming the well region, the following advantages areachieved over the case of forming it by diffusion:

(1) Since the impurity concentration of the well can be readilycontrolled, it can be made uniform.

(2) The junction capacitance in the well surface can be made low, andthe speed of the memory operation becomes high.

(3) Since the impurity concentration in the well surface can be madelow, the breakdown voltage increases.

(4) The control of the threshold voltage is facilitated.

(5) The depth of the well can be precisely adjusted.

Now, the steps of forming N-type well regions by another method will bedescribed with reference to FIGS. 14 to 16.

As shown in FIG. 14, a single crystal of Si is epitaxially grown on thewhole surface of a semiconductor substrate 101 while it is being dopedwith arsenic. The impurity concentration of arsenic is 10¹⁵ cm⁻³. Thus,an N-type well region approximately 3 μm deep is formed uniformly on thesemiconductor substrate 101.

As shown in FIG. 15, in order to form desired N-type well regions, anSiO₂ film 102 and a photoresist film 104 are formed on the N-type wellforming regions. Thereafter, using the SiO₂ film 102 and the photoresistfilm 104 as a mask, boron at an impurity concentration of 2×10¹⁵ cm⁻³ ision-implanted into the surface of the N-type well, whereupon thermaldiffusion is executed to diffuse the boron and to form the same P-typeregions as the semiconductor substrate 101.

As shown in FIG. 16, the SiO₂ film 102 and the photoresist film 104 areremoved to form the desired N-type well regions within the semiconductorsubstrate 101.

It is to be understood that the process for forming the N-type wellregions is not restricted to the two sorts of methods mentioned above,and that other methods may be resorted to if desired. Needless to say,the well regions may be formed by diffusion although, for the reasonsset forth above, epitaxial growth is preferred.

Steps of Forming the Oxide Film and the Oxidation-Resisting Film

As shown in FIG. 17, an SiO₂ film 102 and an insulating film which doesnot let oxygen pass therethrough, that is, an oxidation-resisting film103 are formed on the surfaces of the semi-conductor substrate 101 andthe N-type wells 100. As a preferred material for theoxidation-resisting film 103, a silicon nitride (Si₃ N₄) film is used.

The SiO₂ film 102 is formed to a thickness of approximately 500 Å by thesurface oxidation of the Si substrate for the following reason. When theSi₃ N₄ film 103 is formed directly on the surface of the Si substrate101, a thermal strain is placed on the surface of the Si substrate 101due to the difference between the coefficients of thermal expansion ofthe Si substrate 101 and the Si₃ N₄ film 103. In consequence, crystaldefects are caused in the surface of the Si substrate 101. In order toprevent this drawback, the SiO₂ film 102 is formed on the surface of theSi substrate 101 before the formation of the Si₃ N₄ film 103. On theother hand, the Si₃ N₄ film 103 is used as a mask for the selectiveoxidation of the Si substrate 101 as will be described in detail later,and it is, therefore, formed to a thickness of approximately 1,400 Å by,for example, the CVD (Chemical Vapor Deposition) process.

Steps of Selective Removal of the Oxidation-Resisting Film and IonImplantation

In order to selectively remove the Si₃ N₄ film 103 on those parts of thesurface of the Si substrate 101 in which a comparatively thickinsulating film (that is, a field insulating film) is to be formed, aphotoresist film 104 is first formed on selected areas of the surface ofthe Si₃ N₄ film 103 as an etching mask. Under this state, the exposedparts of the Si₃ N₄ film 103 are removed by, for example, a plasmaetching process capable of precise etching.

Subsequently, in order to prevent layers of the conductivity typeopposite to that of the substrate (i.e., so-called inversion layers)from being formed in the surface parts of the Si substrate 101 in whichthe field insulating film is formed, an impurity of the sameconductivity type as that of the substrate (e.g., a P-type impurity) isintroduced into the Si substrate 101 through the SiO₂ film 102 exposedin the state in which the photoresist film 104 is left as shown in FIG.18. As a process for the introduction of the P-type impurity, ionimplantation is preferred. By way of example, ions of boron, whichprovide a P-type impurity, are implanted into the Si substrate 101 at animplantation energy of 75 keV. The dose of the ions at this time is3×10¹² atoms/cm².

Step of Forming the Field Insulator

A field insulator 105 is formed on the selected areas of the surface ofthe Si substrate 101. More specifically, as shown in FIG. 19, afterremoving the photoresist film 104, the surface of the Si substrate 101is selectively oxidized by thermal oxidation by employing the Si₃ N₄film 103 as a mask, and an SiO₂ film 105 approximately 9,500 Å thick(hereinafter, termed "field SiO₂ film") is formed. During the formationof the field SiO₂ film 105, the ion-implanted boron is brought into theSi substrate 101 by outdiffusion, so that P-type inversion-preventivelayers (not shown) having a predetermined depth are formed directlybelow the field SiO₂ film 105.

Steps of Removing the Oxidation-Resisting Film and the Oxide Film

In order to expose those parts of the surface of the Si substrate 101which are not covered with the field SiO₂ film 105, the Si₃ N₄ film 103is removed with, for example, a hot phosphoric acid (H₃ PO₄) liquid. TheSiO₂ film 102 is subsequently removed with, for example, a hydrofluoricacid (HF) liquid, to expose the selected areas of the surface of the Sisubstrate 101 as shown in FIG. 20.

Step of Forming the First Gate Insulating Film

In order to obtain the dielectric layers of the capacitors C_(S) in thememory cells M-CEL, the exposed surfaces of the Si substrate 101 and theN-type wells 100 are formed with a first gate insulating film 106 asshown in FIG. 21. That is, the exposed surfaces of the Si substrate 101and the N-type wells are thermally oxidized, whereby the first gateinsulating film 106 with a thickness of approximately 430 Å thick isformed in the surfaces. Accordingly, the first gate insulating film 106is made of SiO₂.

Step of Depositing the First Conductor Layer

A first conductor layer 107 for use as one electrode of the capacitorC_(S) in each memory cell is formed on the whole surface of theresultant Si substrate 101 as shown in FIG. 22. More specifically, apolycrystalline silicon layer, for example, is formed on the wholesurface of the Si substrate 101 as the first conductor layer 107 by theCVD process. The thickness of the polycrystalline silicon layer isapproximately 4,000 Å. Subsequently, in order to lower the resistance ofthe polycrystalline silicon layer 107, an N-type impurity, e.g.,phosphorus is introduced into the polycrystalline silicon layer 107 bydiffusion. As a result, the sheet resistance of the polycrystallinesilicon layer 107 becomes approximately 16Ω per square.

Step of Selective Removal of the First Conductor Layer

In order to form the first conductor layer or first polycrystallinesilicon layer 107 into a predetermined electrode shape, the firstpolycrystalline silicon layer 107 is selectively removed by photoetchingso as to form electrodes 108 as shown in FIG. 23. Plasma etching, whichis capable of precise etching, is suitable as a process for selectivelyremoving the first polycrystalline silicon layer 107. Subsequently, theexposed parts of the first gate SiO₂ film 106 are also etched topartially expose the surface of the N-type wells 100.

Step of Forming the Second Gate Insulating Film

In order to obtain the gate insulating films of the MOSFETs in thememory array M-CEL, the dummy array D-CEL and the peripheral circuitportion, the exposed surfaces of the Si substrate 101 and the N-typewells 100 are covered with a second gate insulating film 109 as shown inFIG. 24. That is, the exposed surfaces of the Si substrate 101 and theN-type wells 100 are thermally oxidized, whereby the second gateinsulating film 109 with a thickness of approximately 530 Å is formed inthe surfaces. Accordingly, the second gate insulating film 109 is madeof SiO₂. Simultaneously with the formation of the second gate insulatingfilm or second gate SiO₂ film 109, the surfaces of the electrodes 108made of the first polycrystalline silicon are oxidized and are formedwith SiO₂ films 110 which are approximately 2,200 Å thick. The SiO₂ film110 serves for the inter-layer insulation between the electrode 108 andan electrode made of a second polycrystalline silicon layer to bedescribed later.

Step of Ion Implantation for Controlling Threshold Voltage

As shown in FIG. 25, in order to control the threshold voltage of theN-MOS, a P-type impurity is introduced into that part of the surface ofthe Si substrate 101 in which the N-MOS is to be formed by ionimplantation employing a photoresist film 104 over the surfaces of theN-type wells as a mask for the ion implantation. Boron, for example, isused as the P-type impurity. It is preferable that the implantationenergy is 30 keV and that the does of ions is 4.5×10¹¹ atoms/cm².

Step of Depositing the Second Conductor Layer

A second conductor layer 113 is formed on the whole surface of theresultant Si substrate 101 in order to use it as the gate electrodes forall the MOSFETs and the wiring layers. More specifically, as shown inFIG. 26, a polycrystalline silicon layer, for example, is formed on theentire surface of the Si substrate 101 as the second conductor layer 113by a CVD process. The thickness of the polycrystalline silicon layer 113is approximately 3,500 Å. Subsequently, in order to lower the resistanceof the polycrystalline silicon layer 113, an N-type impurity, e.g.,phosphorus is introduced into this polycrystalline silicon layer bydiffusion. As a result, the sheet resistance of the polycrystallinesilicon layer 113 becomes approximately 10Ω per square.

Step of Selective Removal of the Second Conductor Layer

The second conductor layer or second polycrystalline silicon layer 113is selectively removed by photoetching in order to form it into apredetermined electrode or wiring shape. That is, as partially shown inFIG. 27, the photoetched silicon layer 113 forms the word lines WL₁₋₁-WL₁₋₆, as well as dummy word lines DWL₁₋₁ and DWL₁₋₂ and control signallines φ_(dc) -L1 and φ_(dc) -L2 shown in FIG. 9D. Further, the exposedsecond gate SiO₂ film 109 is removed to expose the surfaces of the Sisubstrate 101 and the N-type wells 100.

Step of Surface Passivation

In order to prevent the contamination of surface parts in which thesource regions and drain regions of the MOSFETs are to be formed, theexposed surfaces of the Si substrate 101 and the N-type wells 100 arecovered with an SiO₂ film 115 which is approximately 100 Å thick by thethermal oxidation thereof as illustrated in FIG. 28. Simultaneously withthe formation of the SiO₂ film 115, the surfaces of the word lines WL₁₋₁-WL₁₋₆, dummy word lines DWL₁₋₁ and DWL₁₋₂, control signal lines φ_(dc)-L1 and φ_(dc) -L2, and the gate electrodes of the complementary MOSFETswhich are made of the second polycrystalline silicon are oxidized, withthe result that the surfaces are covered with an SiO₂ film 116 about 300Å thick, as also shown in FIG. 28.

Step of Forming Source and Drain Regions

In order to form the source and drain regions of the N-MOS in theselected portions of the Si substrate 101, as illustrated in FIG. 29, amask for ion implantation, e.g., CVD SiO₂ film 119, is formed on theN-type wells 100. An N-type impurity, e.g., arsenic, is introduced intothe Si substrate 101 through those areas of the SiO₂ film 115 which arenot covered with the CVD SiO₂ film 119. As a process for introducing theN-type impurity, ion implantation is preferred. By way of example,arsenic ions are implanted into the Si substrate 101 at an implantationenergy of 80 keV. The dose of the ions at this time is 1×10¹⁶ atoms/cm².By subsequently performing a heat treatment, the ion-implanted arsenicimpurity is subjected to outdiffusion, and N⁺ -type semiconductorregions 120 and 121 having a predetermined depth are formed. These N⁺-type semiconductor regions 120 and 121 form the source and drainregions.

Next, in order to form the source and drain regions of the P-MOS in theselected parts of the N-type wells 100, as illustrated in FIG. 30, amask for ion implantation, e.g., CVD SiO₂ film 119, is formed on thesurface of the resultant Si substrate 101 except for the N-type wells100. A P-type impurity, e.g., boron, is introduced into the N-type wellsthrough the SiO₂ film 115 overlying the N-type wells 100 by ionimplantation. By way of example, boron ions are implanted into theN-type wells at an implantation energy of 80 keV. The dose of the ionsat this time is 3×10¹⁵ atoms/cm².

By subsequently performing a heat treatment, the ion-implanted boronimpurity is subjected to outdiffusion, and P⁺ -type semiconductorregions 122-127 having a predetermined depth are formed. These P⁺ -typesemiconductor regions 122-127 form the source and drain regions.

It should be noted that the sources and drains of the P-MOS arepreferably formed after the sources and drains of the N-type MISFETs sothat the heat treatment step need only be carried out once. This willprevent the boron from diffusing more than is necessary.

Contact Hole Forming Step (1)

A contact hole for the connection between the first conductor layer orfirst polycrystalline silicon layer 108 and a third conductor layer tobe discussed later is formed in the SiO₂ film 110. That is, as shown inFIG. 31, a contact hole CH₁₀₁ is formed in the selected part of the SiO₂film 110 by employing a photoresist film (not shown) as a mask. Thiscontact hole CH₁₀₁ corresponds to the contact hole CH₀ shown in FIG. 9A.

The reason why only the contact hole CH₁₀₁ for the connection betweenthe first polycrystalline silicon layer 108 and the third conductorlayer is formed is as follows. As stated before, the thickness of theSiO₂ film 110 formed in the surface of the first polycrystalline siliconlayer 108 is 300 Å. On the other hand, the thickness of the SiO₂ film115 formed in the surfaces of the Si substrate 101 and the N-type wells100 is 100 Å. Accordingly, when these SiO₂ films 110 and 115 are etchedat the same time, there is the possibility that the SiO₂ film 115 willbe overetched before the first polycrystalline silicon film 108 iscompletely exposed. Therefore, in order to prevent this drawback, thecontact hole CH₁₀₁ is formed independently as stated above.

Contact Hole Forming Step (2)

Contact holes for the connections between the source and drain regionsand the third conductor layer are formed in the SiO₂ film 115. That is,using a predetermined mask, contact holes CH₁₀₂ -CH₁₀₇ are formed asshown in FIG. 32 by the selective etching of the SiO₂ film 115. Thecontact hole CH₁₀₂ corresponds to the contact hole CH₁ in FIG. 9A.

It should be noted that although the mask includes an opening at alocation corresponding to the contact hole CH₁₀₁, the overetch of theSiO₂ film 110 in the contact hole CH₁₀₁ does not create a problem inactual practice.

Step of Forming the Inter-Layer Insulating Film

An inter-layer insulating film is formed on the whole surface of theresultant Si substrate 101. That is, as illustrated in FIG. 33, aninter-layer insulating film 118, for example, a phosphosilicate glass(PSG) film, which is approximately 8,000 Å thick is formed on the entiresurface of the Si substrate 101. This PSG film 118 serves also as agetter for sodium ions which affect the characteristics of the MOS.

Contact Hole Forming Step (3)

Contact holes are formed in the PSG film 118 for the connections betweenthe second polycrystalline silicon layer and the third conductor layeras well as between the source and drain regions and the third conductorlayer.

As shown in FIG. 34, the PSG film 118 is selectively etched to formcontact holes CH₁₀₁ -CH₁₀₇ at the same locations as shown in FIG. 32. Amask which is used in forming these contact holes CH₁₀₁ -CH₁₀₇ is thesame as the mask used for forming the contact holes CH₁₀₁ -CH₁₀₇ in thecontact hole forming step (2). Subsequently, the PSG film 118 isheat-treated at a temperature of about 1,000° C. in order to flatten it.

It is also possible that the formation of the contact holes into theSiO₂ film 115 explained in the contact hole forming step (2) is achievedsimultaneously with the formation of the contact holes into the PSG film118. However, while the contact holes in the SiO₂ film 115 are beingformed, the PSG film 118 is also etched. In other words, an overetch ofthe PSG film 118 occurs. In order to prevent the overetch, accordingly,it is preferable that the formation of the contact holes into the PSGfilm 118 and the formation of the contact holes into the SiO₂ film 115are separately executed as described above.

Step of Forming the Third Conductor Layer

In order to form the power supply lines V_(SS) -L and the data linesDL₁₋₁, DL₁₋₁, DL₁₋₂ shown in FIG. 9A, the third conductor layer, e.g.,an aluminum layer which is approximately 12,000 Å thick, is first formedon the whole surface of the resultant Si substrate 101. Subsequently,this aluminum layer is selectively etched to form a power supply lineV_(SS) -L, a data line DL₁₋₁ and a wiring layer 127, as illustrated inFIG. 35.

The D-RAM according to the present invention as thus far described hasthe following advantages:

(1) Since a plurality of N-type well regions formed by an identicalprocess are disposed in a P-type semiconductor substrate, and P-channelMISFETs serving as memory cells and a pair of P-channel FETs of acomplementary sense amplifier are formed in the surfaces of therespective well regions, memory cells which are substantially immune toα-particles and a stable, high speed sense amplifier are simultaneouslyobtained. In this regard, by constructing the memory cell of P-MOSFETsand varying a word voltate within a range between a power source voltageV_(cc) and (V_(cc) -|V_(thp) |), the selection of information "1" or "0"is permitted, and a memory capable of operating at high speed isobtained.

(2) Since a complementary sense amplifier is connected to folded datalines, an areal margin in layout which is approximately double thatfound in prior art devices is obtained. Hence, a high packaging densitybecomes possible.

(3) Since means for precharging folded data lines to a potentialintermediate between the logics "1" and "0" of a memory cell isprovided, the change of the potential of a data line equal to half ofthe potential difference between the logics "1" and "0" determines theread-out time. Hence, a memory of high speed and low power dissipationis obtained.

In addition, the coupling noise of a word line and the data line arecancelled because plus and minus noise develop in the folded data linesrespectively.

Further, since the data line is precharged with a reference potential ata level intermediate between the logics "1" and "0" of the memory cell,a dummy cell can be dispensed with, and a memory of small chip area isobtained.

(4) Since the starting times of the positive feedback operation of theP-channel FET pair of a sense amplifier and the positive feedbackoperation of the N-channel FET pair thereof are made different,through-current does not flow, and a memory of low power dissipation isobtained.

(5) Since the P-channel FET pair and N-channel FET pair of acomplementary sense amplifier are arranged at both the ends of a memoryarray, the layout within a chip can be separated into the group ofP-channel FETs and the group of N-channel FETs, and hence, efficientpackaging is permitted.

(6) Since folded data lines are formed of Al, the wiring resistance isvery low, and an operation of high reliability is permitted.

(7) Since an N-type well region for forming memory cells therein is madeof an epitaxial layer, a uniform well having a desired impurityconcentration can be obtained. Therefore, a threshold voltage can becontrolled and a junction capacitance can be made lower than in a casewhere diffusion is used. This improves the high speed capabilities ofthe memory. In addition, the surface impurity concentration of the wellcan be made lower than in the case of diffusion, so that a memory havingbreakdown voltage is obtained.

(8) Since a plurality of N-type well regions are formed with wellbiasing wirings in parallel with data lines, well voltages becomesubstantially uniform and well resistances can be made low. Thus, amemory is produced which is relatively immune to noise.

(9) When the D-RAM is laid out so as to isolate a well region forforming memory cells therein and a well region for forming a senseamplifier therein, noise which has developed in the sense amplifier hasno influence on the memory cells. Hence, an operation of highreliability becomes possible.

It is to be understood that the above-described arrangements are simpleillustrative of the application of the principles of the invention.Numerous other arrangements may be readily devised by those skilled inthe art which embody the principles of the invention and fall within itsspirit and scope.

We claim:
 1. A semiconductor memory comprising:a plurality of pairs ofdata lines; differential amplifiers, each of which amplifies adifference between signal levels appearing on each pair of data lines;word lines, each of which is arranged so as to intersect with each pairof data lines; and precharging circuits, each of which sets each pair ofdata lines at a potential intermediate between binary signal levels tobe stored in memory cells, before operation of the correspondingdifferential amplifier is started.